1. Field of the Invention
This invention relates to turbine power plants, and in particular, to a hydraulic arrangement for turning the rotor shaft of an axial flow turbine apparatus.
2. Description of the Prior Art
During periods of turbine inoperability or just prior to the starting of the turbine after a long down period, it is the practice in the art to slowly roll the steam turbine rotor in order to minimize the possibility of distortion thereto due to uneven cooling or heating. For this purpose there is usually provided on the turbine rotor shaft a large turning gear. Associated with the turning gear through a suitable speed reducer and gear drive arrangement is an electric motor.
It has been found that the starting torque required to rotate the turbine shaft from rest is much greater than the torque required to keep the shaft in rotation. The electric motor drives which are utilized for present turbine turning gear systems are therefore sized for the maximum torque requirement at starting. These motor drives, however, operate much below their capacity once the turbine rotor has begun to roll and during the slow rolling operation the major portion of the power produced is utilized to overcome the frictional losses in the gear train.
The prior art has reduced the starting torque requirements and turning gear power unit sizes somewhat through the utilization of bearing hydrostatic lifts. These bearing lift systems introduce high pressure oil directly below the turbine shaft in the bearings to reduce the torque required for rotation. The turning gear torque capacity required is still large however, for such contingencies as seal rubs or bearing wipes.
Most electric motor driven systems are operable only at constant speed and to achieve high starting torques, low turning gear speed results. At low turning gear speed, mixed film lubrication of the bearings may result in permanent bearing damage. Therefore, some electric motor driven systems utilize two-speed transmissions to develop the high starting torques necessary initially and to maintain high enough running speeds so as to obtain full film bearing oil lubrication. However, such arrangements are expensive.
In sum, the present electric motor driven systems for providing motive power for the turning of steam turbine rotors leave much to be desired. For example, the requirement of high starting torque coupled with the constant speed operation of most electric motor systems results in a high power loss in the speed reducing arrangement which maintains the rotor rolling after initial start.
The gear train and the speed reducing drives are very expensive. The turning gear itself is costly, subject to wear, and imposes high windage loss on the turbine and oil system when the turbine is at speed. Further, the high in-rush current to the turning gear motor leads to infrequent starting. Of course, overload torque may result in permanent motor coil damage.
It is thus seen that the prior art system utilizing constant speed electric motors associated with speed reducers and gear drives in order to provide slow rolling of steam turbine rotors is both inefficient and uneconomical.